A flat is a set of hyperplanes, maximal with respect to the property that they contain a given subspace. In this package, flats are treated as lists of indices of hyperplanes in the arrangement.
The source of this document is in /build/reproducible-path/macaulay2-1.25.06+ds/M2/Macaulay2/packages/HyperplaneArrangements.m2:2322:0.